The Universal Mobile Telecommunication System (UMTS) is one of the 3G mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a wireless device such as a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB or eNB). An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. In LTE, the eNodeB manages the radio resources in the cells, and is directly connected to the Core Network (CN), as well as to neighboring eNodeBs via an X2 interface.
FIG. 1 illustrates a typical E-UTRAN comprising a UE 150 wirelessly connected to a serving eNodeB 110a. The serving eNodeB 110a is also connected to neighbouring eNodeBs 110b and 110c, via the X2 interface.
LTE Overview
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink (DL) and Discrete Fourier Transform (DFT)-spread OFDM in the uplink (UL). The basic LTE DL physical resource can thus be seen as a time-frequency grid 50 as illustrated in FIG. 2a, where each resource element 210 corresponds to one OFDM subcarrier 220 during one OFDM symbol interval 230.
In the time domain, LTE DL transmissions are organized into radio frames of 10 ms, each radio frame 270 consisting of ten equally-sized subframes 250 with a length of 1 ms (see FIG. 2b). Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB), where a RB corresponds to one slot (0.5 ms) in the time domain—a time slot 260—and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent RB in time direction (1 ms) is known as a RB pair. RBs are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of Virtual RBs (VRB) and Physical RBs (PRB) has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRBs are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain, thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
DL transmissions are dynamically scheduled, i.e., in each subframe 250 the RBS transmits control information about to which terminals data is transmitted and upon which RBs the data is transmitted in the current DL subframe. This control signaling is typically transmitted in the first or in the two, three or four first OFDM symbols 230 in each subframe 250. The number n=1, 2, 3 or 4 indicates the number of OFDM symbols used for control signaling and is known as the Control Format Indicator (CFI). The DL subframe 250 also contains common reference symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information. A DL subframe 250 with CFI=3, i.e. three OFDM symbols 280 used for control, is illustrated in FIG. 2c. 
Carrier Aggregation
The LTE Rel-10 specifications have been standardized, supporting Component Carrier (CC) bandwidths up to 20 MHz, which is the maximal LTE Rel-8 carrier bandwidth. An LTE Rel-10 operation wider than 20 MHz is possible and appear as a number of LTE CCs to an LTE Rel-10 terminal. The straightforward way to obtain bandwidths wider than 20 MHz is by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC may have the same structure as a Rel-8 carrier. CA is illustrated in FIG. 2d, where five CC of 20 MHz, 295, are aggregated to a total bandwidth of 100 MHz, 290. The Rel-10 standard support up to 5 aggregated CCs 295 where each CC is limited in the radio frequency specifications to have one out of six bandwidths, namely 6, 15, 25, 50, 75 or 100 RB corresponding to 1.4, 3, 5, 10, 15, and 20 MHz respectively.
The number of aggregated CCs 295 as well as the bandwidth of the individual CCs may be different for UL and DL. A symmetric configuration refers to the case where the number of CCs in DL and UL is the same whereas an asymmetric configuration refers to the case that the number of CCs is different in DL and UL. It is important to note that the number of CCs configured in the network may be different from the number of CCs seen by a terminal. A terminal may for example support more DL CCs than UL CCs, even though the network offers the same number of UL and DL CCs.
CCs are also referred to as cells or serving cells. More specifically, in an LTE network the cells aggregated by a terminal are denoted primary Serving Cell (PCell) and secondary Serving Cells (SCells). The term serving cell comprises both PCell and SCells. All UEs have one PCell and the choice of a UEs PCell is terminal specific. The PCell is considered more important, i.e., vital control signaling and other important signaling is typically handled via the PCell. UL control signaling is always sent on a UEs PCell. The CC configured as the PCell is the primary CC whereas all other CCs are SCells.
During initial access a LTE Rel-10 terminal behaves similar to a LTE Rel-8 terminal. However, upon successful connection to the network a Rel-10 terminal may—depending on its own capabilities and on the network—be configured with additional serving cells in the UL and DL. Configuration is based on Radio Resource Control (RRC). Due to the heavy signaling and rather slow speed of RRC signaling it is envisioned that a terminal may be configured with multiple serving cells even though not all of them are currently used.
SCell Activation and Deactivation
With the concept of SCells, additional bandwidth resources could be configured and deconfigured dynamically. The configuration and deconfiguration of cells are signaled by the eNodeB and performed with RRC signaling which is heavy signaling and slow. Since RRC signaling is heavy and slow the concept of activation and deactivation was introduced for SCells. The eNodeB has the possibility to deactivate a UE's serving cells. The eNodeB decides to deactivate serving cells that the UE should not use for the moment. Activation and deactivation is performed with Medium Access Control (MAC) signaling which is faster. The activation and deactivation procedure is described in detail in section 5.13 of 3GPP TS 36.321; Medium Access Control (MAC) protocol specification, V11.0.0 (2012-09). Each SCell is configured with a SCellIndex, which is an identifier or a so called Cell Index which is unique among all serving cells configured for this UE. The PCell always have Cell Index 0 and SCells can have an integer cell index of 1 to 7.
The Rel-10 Activation/Deactivation MAC Control Element (CE) is defined in section 6.1.3.8 of 3GPP TS 36.321. The Activation/Deactivation MAC CE consists of a single octet containing seven C-fields and one R-field. Each C-field corresponds to a specific SCellIndex and indicates whether the specific SCell is activated or deactivated. The UE will ignore all C-fields associated with Cell indices not being configured. The Activation/Deactivation MAC CE always indicates the activation status of all configured SCells, meaning that if the eNodeB wants to activate one SCell it has to include all configured SCells, setting them to activated or deactivated even if their status has not changed.
If a UE's serving cell is activated it would imply that the UE has to monitor PDCCH and PDSCH for that serving cell. This implies e.g. a wider receiver bandwidth and higher sampling rates resulting in high power consumption compared to if that serving cell would have been deactivated.
Sounding Reference Signal Transmissions
In LTE the serving eNodeB can configure a UE to transmit Sounding Reference Signals (SRS) in the UL. The SRS is a physical signal and more specifically it is a type of reference signal.
The aim of SRS is to enable the eNodeB to determine the UL channel status or radio link quality in the frequency domain. The SRS may also be used for performing certain type of UE and eNodeB radio measurements, e.g. for UL positioning measurements such as UE Rx-Tx time difference, eNodeB Rx-Tx time difference, Timing Advance (TA), and angle of arrival (AoA).
The SRS is transmitted periodically in the time domain. The subframes used for SRS transmission in time domain and their bandwidths are semi-statically configured using RRC signaling. The SRS are transmitted in the last Single Carrier Frequency Division Multiple Access (SC-FDMA) symbol of a subframe when configured for SRS transmissions. In the frequency domain SRS can be of different bandwidths according to their semi-static configurations.
Channel State Information Reporting
The UE performs and report channel state information (CSI) measurements for all the serving cells including PCell and SCell(s) to the network node to facilitate e.g. DL scheduling, link adaptation, and antenna mode selection performed by the network. In LTE, CSI measurements comprises:                Rank indication (RI): RI is used to indicate recommended number of layers for DL transmission using DL multi antenna scheme.        Precoder matrix indication (PMI): PMI indicates the recommended precoder matrix that must be used for DL transmission.        Channel quality indication (CQI): CQI indicates the highest modulation and coding (MCS) scheme or transport format that can be used for DL transmission.        
The CSI measurements and reporting are configured at the UE by its serving node. In LTE the network node (e.g. eNodeB) can configure the UE to report CSI using periodic and/or aperiodic mechanisms. The periodicity is also configured by the network. The CSI measurements can be sent by the UE to the eNodeB on a control channel such as Physical UL Control Channel (PUCCH), or on a data channel such as Physical UL Shared Channel (PUSCH). In case of multi-carrier the CSI feedback information for SCell is also sent on the UL PCell.
The UE sends valid feedback information such as valid CQI associated with the activated SCell only when the SCell is activated. This feedback information may be used by the network to determine when exactly the SCell is activated. The network may start scheduling the data on the SCell as soon as it is activated. However the exact activation time is uncertain since it depends upon the level of UE synchronization with the deactivated SCell. Upon reception of the SCell activation command from the network, the UE may take between 8 ms to 34 ms to activate the deactivated SCell.
Therefore in existing solutions, the eNodeB may need to wait before using the new SCell until the complete activation time has passed, e.g. up to 34 ms. This may therefore lead to significant performance degradation as it e.g. creates an unnecessary interruption in communication in cases when the UE can actually activate the SCell much faster than in 34 ms. In addition to reduced performance, it may also cause buffer overflow and wastage of resources as they cannot be assigned.